1. Field of the Invention
The present invention relates to a multiple-antenna wireless communication system, and more particularly, to a method for efficiently constructing a codebook subset to reduce feedback overhead for precoding, and a method for transmitting and receiving signals using the same.
2. Discussion of the Related Art
With the popularization of information communication services, the appearance of various multimedia services, and the provision of high-quality services, demand for a fast wireless communication service has rapidly increased. To actively cope with such demand, first of all the capacity of a communication system should be increased. To increase communication capacity in a wireless communication environment, there can be considered a method for finding new available frequency bands and a method for increasing efficiency of restricted resources. As to the latter method, a multiple-input multiple-output (MIMO) technology has recently attracted much attention and has been actively developed. The MIMO technology obtains a diversity gain by equipping a transmitting side and a receiving side with a plurality of antennas to additionally ensure space for utilizing resources, or increases transmission capacity by transmitting data in parallel through the respective antennas.
In brief, MIMO refers to a method capable of improving transmission/reception data efficiency using multiple transmit antennas and multiple receive antennas, instead of a conventional method employing one transmit antenna and one receive antenna. That is, MIMO is a technology utilizing multiple antennas in a transmitting side or a receiving side of a wireless communication system to increase capacity or improve performance. Hereinafter, MIMO is referred to as “multiple antenna(s)”.
The multiple antenna technology is a technology for restoring data by collecting pieces of data received through several antennas, instead of depending on a single antenna path, in order to receive one overall message. The multiple antenna technology achieves an improvement in data transmission rate at a specific range or an increase in a system range for a specific data transmission rate.
It is predicted that an efficient multiple antenna technology is indispensable because next-generation mobile communication demands a much higher data transmission rate than conventional mobile communication. Under such circumstances, the MIMO communication technology is a next-generation mobile communication technology which can be widely applied to mobile communication terminals and repeaters, and is drawing attention as a technique to overcome limitations in mobile communication capacity, which is increasingly limited due to expansion of data communication.
Meanwhile, among various technologies for improving transmission efficiency which are currently being studied, the MIMO technology using multiple antennas in both a transmitter and a receiver is most noteworthy as a method of remarkably improving communication capacity and transmission/reception performance without increasing additional frequency allocation or power consumption.
FIG. 1 is a view illustrating a configuration of a general MIMO communication system.
If the numbers of transmit and receive antennas are simultaneously increased to NT and NR, respectively, a theoretical channel transmission capacity is increased in proportion to the number of antennas, unlike the case where only either a transmitter or a receiver uses multiple antennas. Accordingly, a transmission rate can be increased and infrequency efficiency can be remarkably improved. The transmission rate due to an increase in a channel transmission capacity can be theoretically increased by a value obtained by multiplying the following rate increase Ri by a maximum transmission rate Ro when using one antenna.Ri =min(NT,NR)  [Equation 1]
For example, a MIMO communication system using 4 transmit antennas and 4 receive antennas can theoretically obtain 4 times the transmission rate of a single antenna system.
Since an increase in the theoretical capacity of the multiple antenna system was first demonstrated in the mid-1990s, various techniques for substantially improving a data transmission rate have been actively developed. Several of these techniques have already been applied to a variety of wireless communication standards such as the 3rd generation mobile communication and the next-generation wireless local area network.
Research up to now related to multiple antenna technologies has been actively conducted in many aspects, including research into information theory related to the computation of multiple antenna communication in various channel environments and in multiple access environments, research into wireless channel measurement and model introduction of a multiple antenna system, and research into space-time signal processing technologies for improving transmission reliability and a transmission rate.
The multiple antenna technology includes a spatial diversity scheme for increasing transmission reliability using symbols passing through various channel paths and a spatial multiplexing scheme for improving a transmission rate by simultaneously transmitting a plurality of data symbols using a plurality of transmit antennas. Recently, research into a method of combining the above-described schemes to take advantage of the respective schemes has been carried out.
A description of the aforementioned schemes will now be given in detail.
First, the spatial diversity scheme includes a space-time block coding method, and a trellis coding method using both a diversity gain and a coding gain. The trellis coding is generally excellent in terms of the improvement of a bit error rate and the degree of freedom in generating codes but the space-time block coding method is simple in terms of computation. The spatial diversity gain can be obtained up to the extent corresponding to a multiplication (NT×NR) of the number NT of transmit antennas and the number NR of receive antennas.
Second, the spatial multiplexing scheme transmits different data streams through respective transmit antennas. At this time, in a receiver, mutual interference may be generated between data which is simultaneously transmitted from a transmitter. Then the receiver eliminates the interference using proper signal processing methods and receives the data. The receiver used for eliminating the interference includes a maximum likelihood receiver, a zero forcing (ZF) receiver, a minimum means-squared errors (MMSE) receiver, a diagonal Bell laboratories layered space-time (D-BLAST) receiver, and a vertical Bell laboratories layered space-time (V-BLAST) receiver. Especially, if the transmitter could recognize channel information, a singular value decomposition (SVD) method may be used to eliminate the interference.
Third, a combination of the spatial diversity scheme and the spatial multiplexing scheme may be used. If only the spatial diversity gain is obtained, a performance improvement gain caused by an increase in a diversity order is gradually saturated. If only the spatial multiplexing gain is obtained, the transmission reliability of a wireless channel is decreased. Accordingly, studies on a method for solving theses shortcomings and simultaneously obtaining those two gains have been conducted. As a result, a double space-time transmit diversity (double-STTD), and a space-time bit interleaved coded modulation (STBICM) have been used.
In the aforementioned multiple antenna system, the transmitting side precodes transmission data and then transmits the precoded data, and the receiving side receives the data using a precoding matrix (or precoding vector) used in the transmitting side.
Meanwhile, the precoding matrix for performing precoding uses one of precoding matrixes which are specified in the form of codebooks which is prescribed by both the transmitting side and the receiving side. A transmission mode of the transmitting side may be divided into an open-loop transmission mode and a closed-loop transmission mode according to whether the precoding matrix used by the transmitting side demands feedback information from the receiving side.
In the open-loop transmission mode, the transmitting side transmits signals using a precoding matrix without using feedback information from the receiving side. In the closed-loop transmission mode, however, the receiving side indicates a specific precoding matrix(s) among previously specified codebooks according to a receiving signal by feeding back channel information, etc., and the transmitting side transmits signals using the feedback information.
The open-loop transmission mode and the closed-loop transmission mode may have different requirements. However, the open-loop transmission mode and the closed-loop transmission mode do not have separate code subsets according to transmission modes. Therefore, feedback information of the receiving side in the closed-loop transmission mode should use large amount of information in order to indicate a precoding matrix used among the entire precoding matrixes including the codebook and thus overhead may be generated.